Half life in water treatment

It’s the little things that trip you up

There’s a lot of chemistry, physics and maths involved in perfecting your water treatment solution. To make sure you successfully treat and protect your system you need to get the equilibrium right, and this relies on balancing all the appropriate equations – even the little things you may not give much thought to. Missing something like half life out of your planning can have serious negative implications for your water treatment, especially when it comes to complying with regulations such as those for Legionella control.

When dosing your water cooling system with biocides it’s imperative that the concentration is correct and that it remains at a continuous concentration for a suitable period of time. While this sounds simple, it’s easy to forget that any bleed water required to compensate for water that may evaporate out of a system, will take a portion of your biocide with it.

Say you put 10 tonnes of make-up water into your system, and every hour 1 tonne runs off as bleed water, this will determine the half life for your system. So for example, let’s imagine the chemical you are using to meet the Government's Health and Safety Executive (HSE) Legionella control L8 Code of Practice needs to remain at a concentration of 100 parts per million (ppm) for three hours to be successful.

If you just dose 100ppm and walk away, the concentration will gradually fall from the time of dosing and will not remain high enough for long enough as the bleed water will take a portion away with it. This will result in a failure to meet the regulation, making you negligent and leaving you liable.

This is why it’s vital to be aware of half life so that you can increase the dose of your biocide accordingly. Do you know how many hours it would take to reduce a 100ppm dose to 50ppm in your water cooling system?

Although it’s hard to be 100 per cent accurate, you can work out your half life with this simple equation:

If you know your biocide is going to take three hours to be effective, but the half life of your system is one hour you’re going to have to make some adjustments to maintain appropriate concentration. For instance, using our above example of legionella control biocide, to stay at a minimum of 100ppm for long enough you’ll need to dose to 800ppm.

A bit of predictive mathematics goes a long way towards protecting your water system and keeping you compliant. Don’t let a little thing like half life leave you vulnerable to negligence claims - do the maths first!